Protective device for electrostatic sprayer equipment

ABSTRACT

Electrostatic spraying equipment having, in a working area likely to contain inflammable mixtures, at least a terminal part of a high-voltage generator with an electrode for charging a jet of material sprayed from a sprayer. At least one low-voltage line connects the terminal part of the generator to a remotely sited power supply equipped with cut-out device. A protective device designed to prevent repeated arcing in the working area as a result of deterioration of the low-voltage line comprises, in series with the line, a sensing device for sensing a current lower than a predetermined minimum current. A control device responsive to the sensing device are connected to the aforementioned cut-out device so as to operate them when a current in the line lower than the predetermined minimum is sensed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a protective device for electrostatic sprayerequipment comprising, in a working area likely to contain inflammablemixtures, at least a terminal part of a high-voltage generator with anelectrode for electrostatically charging a jet of sprayed material, atleast one low-voltage electrical line connecting the terminal part ofthe high-voltage generator to a remotely sited electrical power supplyequipped with cut-out means.

2. Description of the prior art

The use of high-voltage DC generators for supplying electrostaticsprayer equipment is well known as is the risk of ignition of thesprayed products or the vapors thereof as a result of the accidentalproduction of electrical arcs, for example between the high-voltageelectrode of the sprayer apparatus and the object to be sprayed, whichis generally grounded. The risk is proportional to the quantity ofelectrostatic energy stored in the capacitance consisting of thehigh-voltage electrode, the output terminal of the generator and thecable connecting these. When the generator is sited far from the sprayerapparatus, at a distance of around ten meters, for example, the greaterpart of this capacitance consists of the connecting cable. The risk ofarcing due to the capacitance of the cable is often reduced by insertinga damper resistance between it and the high-voltage electrode of thesprayer apparatus; if the capacitance of said electrode is low thisprotection is sufficient. If not, or in order to take account also ofthe risk of deterioration of the insulation of the high-voltageconnecting cable, there is placed in the high-voltage circuit, generallybetween the ground terminal of the generator and the ground connection,a detector responsive to a current surge or to a rapid increase in thecurrent and which, when it detects an anomaly in the high-voltagecurrent, shuts down the high-voltage generator and connects itshigh-voltage terminal to ground in order to discharge rapidly to groundthe capacitance which is charged to the high voltage. Another proposedsolution is to reduce the capacitance charged to the high voltage byeliminating the connecting cable and juxtaposing the generator and thesprayer apparatus. This solution eliminates the risk associated with thecapacitance of the high-voltage cable but not that associated with thecapacitance of the high-voltage electrode of the sprayer apparatus. Ithas consequently been proposed, in particular in U.S. Pat. No.4,187,527, to proceed as previously and to provide between the groundterminal of the high-voltage generator and the ground connection, in thehigh-voltage circuit, a detector responsive to an increase in thecurrent by shutting down the generator and grounding the high-voltageelectrode. As the generator may be grounded far away from the sprayerapparatus, in an area containing no combustible mixture, thehigh-voltage current increase detector is located in such an area andthus does not present any risk.

Other risks result from this solution, however. Even though thelow-voltage input of the high-voltage generator and the device forgrounding the high-voltage electrode are designed and constructed insuch a way as to eliminate the risk of electrical arcing, it isdifficult to protect effectively the low-voltage cables connecting themto their respective power supplies against this risk without making themstiff and heavy. This is unacceptable when the sprayer apparatus ismobile and light in weight, if it is mounted on a manipulator possessingseveral degrees of freedom, for example. Nor is it possible to limit theenergy available in these low-voltage cables, for example by making themintrinsically safe using Zener barriers, since the power to betransmitted is too high. As this power is generally transmitted as acurrent of approximately one ampere with a voltage of several tens ofvolts, the risk of short-circuiting between the conductors of thelow-voltage cables carrying these currents is easily eliminated by usingmaximum current circuit-breakers and/or fast-blow fuses. This does notapply to the risk of intermittent interruption of the circuitsconstituted by these cables, for example as a result of the rupture of aconductor due to fatigue resulting from the alternate bending imposed onit when manipulated. These intermittent interruptions do not result incurrent surges which can be detected by the aforementioned devices butthey can nevertheless produce electrical arcs able to ignite inflammablemixtures as a result of the self-inductance and/or capacitance of thecircuits in question.

SUMMARY OF THE INVENTION

In electrostatic sprayer equipment comprising a high-voltage generator,a sprayer, an electrode adapted to charge a jet of material sprayed fromsaid sprayer, an electrical power supply adapted to be sited remotely ofsaid generator, cut-out means equipping said electrical power supply,and at least one low-voltage line adapted to connect said generator tosaid electrical power supply, the invention consists in a protectivedevice which comprises means connected in series with said line andadapted to sense the electric current therein, and control means whichare responsive to said sensing means, connected to said cut-out meansand adapted to operate said cut-out means when said sensing means sensea current lower than a predetermined minimum current.

Thus when a rupture in a conductor of the power supply line firstappears, with intermittent breaks in continuity, the resultingreductions in the current, sensed by the sensing means, lead to theintervention of the control means which activate the cut-out means toprevent the occurrence of repeated arcing.

The electrical power supply is preferably adapted to supply aunidirectional electric current. The sensing means may then comprise athreshold comparator and a resistor adapted to be connected in serieswith the line and to an input of the threshold comparator.

A preferred embodiment comprises short-circuiting means shunting theline and adapted to be operated by the control means in conjunction withthe cut-out means, so as to discharge the line when the supply is cutoff.

When the electrode for charging the jet of sprayed material is equippedwith electrically controlled grounding means, the control line for thegrounding means is protected by a device analogous to that of the linesupplying the terminal part of the high-voltage generator.

Other objects and advantages will appear from the following descriptionof an example of the invention, when considered in connection with theaccompanying drawings, and the novel features will be particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation showing the organization of adevice in accordance with the invention.

FIG. 2 is a circuit schematic of one embodiment of the device inaccordance with the invention.

GENERAL DESCRIPTION OF THE PREFERRED EMBODIMENT

In the embodiment shown in FIG. 1, opposite an object 1 to be sprayedand which is grounded through a line 1a, there is disposed electrostaticsprayer apparatus 2 supplied with a high DC voltage by a generator 3which is itself grounded via a line 3a and receives power from a powersupply 4 via a low-voltage line 4a. A device 5 for rapidly discharging ahigh-voltage electrode 2a of the sprayer apparatus 2 is grounded via therapid discharge device 5 a line 5a and receives electrical power from apower supply 6 via a low-voltage line 6a. The low-voltage power supplies4 and 6 receive power from a voltage source (not shown) via a line 7.

A first sensor 10 senses any variation in or hazardous value of thehigh-voltage current in the grounding line 3a for the generator 3. Thisdevice 10 controls the low-voltage supply 4 of the generator 3 via aline 10a and the low-voltage supply 6 of the fast discharge device 5 viaa line 10b, in the known manner. A second sensor 11 senses any variationin or hazardous value of the low-voltage current in the line 4a andcontrols the low-voltage power supply 4 via a line 11a.

In a similar manner, a third sensor device 12 senses any variation in orhazardous value of the low-voltage current in the line 6a and controlsthe low-voltage power supply 6 via a line 12a.

The object to be sprayed, the sprayer apparatus, the high-voltagegenerator and the device for rapidly discharging the high-voltageelectrode are situated in an area 15 which may contain inflammablemixtures, whereas the other component parts of the installation such asthe power supplies and the sensors are situated in an area 16 which doesnot contain any such mixtures, the connections between these two groupsof components being made by the ground lines 3a and 5a and thelow-voltage lines 4a and 6a.

OPERATION

If the object 1 to be sprayed comes too close to the high-voltageelectrode of the sprayer 2 there results an increase in the high-voltagecurrent in the line 3a which is sensed, in a manner known per se, by thesensor 10 which, over the line 10a, shuts down the low-voltage powersupply 4 of the high-voltage generator 3 and causes, via the line 10b,the low-voltage power supply 6 and the low-voltage line 6a, thegrounding of the high-voltage electrode 2a of the sprayer 2 via thedischarge device 5 and line 5a. If the low-voltage power line 4asupplying the high-voltage generator 3 and/or the low-voltage power line6a supplying the discharge device is interrupted, as a result ofrepeated movement of the sprayer-generator-discharge device combination,for example, the circuits consisting of these lines 4a and 6a areintermittently interrupted because of these movements of theaforementioned combination and electrical arcs may be struck at theposition of any interruptions situated in the danger area 15. Thesensors 11 and 12 respectively sense these interruptions, and inparticular the sudden drops in the current corresponding thereto, andrespectively shut down the low-voltage power supplies 4 and 6, wherenecessary short-circuiting the conductors of the lines 4a and 6a so asto dissipate outside the danger area 15 any energy stored therein ininductive or capacitive form.

DETAILED DESCRIPTION OF SENSOR

Referring to FIG. 2, the high-voltage generator 3 consists primarily ofa class C oscillator 3j, a high-voltage transformer 3k and a voltagemultiplier 3l. It applies a high DC voltage to the terminal 3m through adamper resistor 3n. It is grounded via a line 3a. A capacitor 3pproduces the current spikes necessary for the oscillator to function.

The oscillator 3j is supplied with a DC voltage, affected by pulses to agreater or lesser extent, by the power supply 4 via the 2-wire line 4a.The power supply 4 primarily comprises a DC voltage source, symbolicallyrepresented here as a battery 4b, a ballast transistor 4c, a capacitor4d, a discharge transistor 4e and its protective resistor 4f. The powersupply also comprises a Zener diode 4g and a resistor 4h for stabilizingthe supply to the device 11. A fast-blow fuse 4i protects the line 4aagainst current surges, due to a short-circuit between its conductors,for example. The device 11 comprises a sensing circuit consisting of aresistor 11b and a coupling capacitor 11c for the AC component, a filterconsisting of a resistor 11d and a capacitor 11e, and an operationalamplifier 11f which compares the voltage obtained across the capacitor11e with a DC reference voltage provided by a potentiometer 11g,supplied through the resistor 4h and the Zener diode 4g. A flip-flop 20is driven by the operational amplifier 11f and where applicable by otheramplifiers 11h, 11i controlled by other (not shown) sensing devices.This flip-flop 20 controls via a line 20a the transistor 4c supplyingthe oscillator 3j and via a line 20b the transistor 4e for dischargingthe capacitors 3p and 4d as well as the capacitance of the line 4a. Viaother lines 20c and 20d, for example, flip flop 20 operates likewise onthe power supply 6 of the device 5 for discharging the high-voltageelectrode 2a of the sprayer 2. An external control line 20e provides fordeliberate disabling of the flip-flop 20 where variation of the currentin the line 4a is expected, for example on starting up, adjusting orshutting down the high-voltage generator 3. A line 20f is provided foractivating an indicator device to show that the device has operated.

OPERATION

When the (possibly pulsed) DC current in the line 4a is normal, thecomparison of the voltages sensed by the circuits 11b, 11c, 11d, 11ewith the DC voltage set by the potentiometer 11g maintains the amplifier11f and the flip-flop 20 in a state such that the power supply 4 maycontinue to supply the normal current. If the line 4a is subject to oneor more interruptions between the device 11 and the generator 3, thereoccur rapid and significant reductions in the current resulting inreductions in the voltage applied to the amplifier 11f. These voltagedrops, amplified by the amplifier, cause the flip-flop 20 to changestate. This in turn shuts down the power supply 4 by switching off thetransistor 4c via the line 20a and discharges the capacitors 3p and 4dand the capacitance of the line 4a by switching on the transistor 4e. Bymeans of lines 20c and 20d, this change of state may also bring aboutsimilar effects at the power supply 6 for the discharge device 5. Italso activates an alarm signal over the line 20f. The components of theelectrical circuits are selected for the fastest possible response timewhile eliminating spurious reaction to interference in the low-voltagecircuit under normal operating conditions. In this way response times ofthe order of a few milliseconds have been routinely obtained.

It will be understood that various changes in the details, materials andarrangements of parts which have been herein described and illustratedin order to explain the nature of the invention may be made by thoseskilled in the art within the principle and scope of the invention asexpressed in the appended claims.

We claim:
 1. Electrostatic sprayer equipment comprising:(a) a high-voltage generator; (b) a sprayer operatively connected to said high-voltage generator; (c) an electrode for charging a jet of material from said sprayer; (d) an electrical power supply for supplying power to said high-voltage generator, said electrical power supply being sited remotely from said high-voltage generator; (e) cut-out means for cutting off power from said electrical power supply; (f) at least one low-voltage line for connecting said high-voltage generator to said electrical power supply; and (g) a protective device for operating said cut-out means when a lower than threshold current is sensed in said at least one low-voltage line.
 2. The electrostatic sprayer equipment according to claim 1 wherein said protective device comprises:(a) sensing means connected in series with said at least one low-voltage line for sensing current in said at least one low-voltage line; and (b) control means connected to said cut-out means, said control means being responsive to said sensing means to operate said cut-out means to cut off power from said electrical power supply wherein said cut-out means is operated when said sensing means senses a current lower than said threshold current.
 3. The electrostatic sprayer equipment according to claim 2 wherein said electrical power supply supplies a unidirectional electric current.
 4. The electrostatic sprayer equipment according to claim 3 wherein said sensing means comprises:(a) a threshold comparator having at least one input; and (b) a resistor adapted to be connected in series with said at least one low-voltage line and to said at least one input of said threshold comparator.
 5. The electrostatic sprayer equipment according to claim 2 further comprising short-circuiting means operated by said control means in response to detection of current less than said threshold current for shunting said at least one low-voltage line and effecting safe discharge of capacitance.
 6. The electrostatic sprayer equipment according to claim 3 wherein said control means comprises a flip-flop having a disable input for disabling said flip-flop prior to starting or, during adjusting or shutting down the generator.
 7. The electrostatic sprayer equipment according to claim 3 further comprising:(a) means for grounding said electrode; (b) a low-voltage control line connected to said means for grounding; and (c) means connected to said low-voltage control line and said control means for detecting a minimum current.
 8. Electrostatic sprayer equipment comprising:(a) a sprayer for spraying material; (b) a high-voltage generator operatively connected to said sprayer for charging the material sprayed by said sprayer; (c) an electrical power supply adapted to supply power to said high-voltage generator; (d) at least one low-voltage line connected between said high-voltage generator and said electrical power supply; (e) first means for detecting current in said at least one low-voltage line; and (f) safety means responsive to said first means for detecting to interrupt the flow of power from said electrical power supply to said high-voltage generator when said current in said at least one low-voltage line is lower than a predetermined threshold.
 9. The electrostatic sprayer equipment according to claim 8 further comprising:(a) an electrode for charging a jet of material from said sprayer; (b) grounding means for selectively grounding said electrode; (c) at least one low-voltage control line connected to said grounding means; (d) second means for detecting current in said at least one low-voltage control line, and wherein said safety means is responsive to said second means for detecting to interrupt the flow of power to said at least one low-voltage control line and said at least one low-voltage when said current in said at least one low-voltage control line is lower than a predetermined threshold.
 10. The electrostatic sprayer equipment according to claim 9 wherein said electrical power supply sits remotely from said high-voltage generator.
 11. The electrostatic sprayer equipment according to claim 10 further comprising cut-out means operated by said safety means in response to detection of a current lower than said predetermined threshold for interrupting power from said high-voltage power supply, said cut-out means comprising at least one transistor.
 12. The electrostatic sprayer equipment according to claim 11 further comprising short-circuiting means operated by said safety means in response to detection of current less than said threshold current for shunting said at least one low-voltage line and effecting safe discharge of capacitance.
 13. The electrostatic sprayer equipment according to claim 13 wherein said safety means comprises a flip-flop having a disable input for disabling said flip-flop prior to starting or, during adjusting or shutting down the generator 